Intrauterine growth retardation ocurs when fetal growth is attenuated prior to birth. It is analogous to growth failure during childhood, but because there are so few instances when accurate interval growth is assessed prior to birth, it is usually diagnosed on the basis of birth weight rather than on rate of growth. It is a common disorder with multiple etiologies, occurring in up to 3% of all pregnancies. It is associated with significant morbidities, such as hypoglycemia, intellectual deficit, and permanent short stature. Although many studies have served to delineate the clinical spectrum of IUGR, and relate some of the etiologies and clinical features to outcome, in most cases, the causes of IUGR remain unknown and virtually nothing is known of the precise mechanisms which ultimately produce IUGR in humans. The objective of this project, therefore, is to begin to address the problem of IUGR by identifying abnormalities of the IGF-1 receptor as responsible for IUGR in humans. Data from animal and human studies provide solid evidence that a deficiency of IGF-1 action, either from reduced production of the peptide or dysfunction of the IGF-1 receptor, may lead to IUGR in man.